During the last three decades, captured fisheries production increased from 69 to 93 million tonnes; world aquaculture production increased from 5 to 63 million tonnes. Although 70% of the Earth's surface is covered by water, fish (including shellfish) only represent 6.5% of all protein for human consumption whereas aquaculture represents around 2%. Fish is usually low in saturated fats, carbohydrates and cholesterols and provides not only high value proteins but also a wide range of essential micronutrients, including various vitamins, minerals, and poly-unsaturated omega-3 fatty acids. Thus, even in small quantities, provision of fish can be effective in addressing food and nutritional security among the poor and vulnerable populations around the globe [1].
Current industrial aquaculture farming system is based on open net pen culture. Examples given are typically for the Norwegian salmon industry. The description may just as well apply for salmon farming in other countries, but is to a variable degree relevant to other aquaculture industries. Oxygen is provided through incoming water and fish faeces and carbon dioxide (CO2) and ammonium (NH4) is discharged and carried away by the outflowing water.
The net pen production system leaves the fish population exposed to the open environment. Water that flows through the net pen may carry harmful microorganisms that potentially can infect the fish population. Several naturally occurring microorganisms2 (Vibrio anguillarum, Vibrio salmonicidae, Aeromonas salmonicidae, Moritella viscosa, Infectious Pancreatic Necrosis Virus, Salmonid Alphavirus, Infectious Salmon Anaemia Virus, Piscirickettsia salmonis, Infectious Hematopoietic Necrosis Virus, and many more) can cause disease in farmed salmonids [2]. The fish health status is subject to comprehensive surveillance both internationally (www.oie.int), from National animal health institutes [3] and also by the farming companies. To combat the most prevalent bacterial and viral diseases, pharmaceutical industry have researched and developed vaccines that are common in use. The value of vaccination is undisputed in industry. Harmful microorganisms cannot be eliminated by vaccination, but vaccination immunises the fish and enables it to reject the infection and not develop clinical disease. Far from all harmful organisms can be prevented by vaccination.
Parasites prevalent to wild salmonids such as the sea louse (Lepeophtheirus and Caligus), infect farmed salmonids. The most prevalent and widespread is the Salmon louse (Lepeophtheirus salmonis). Once clinical disease is established in one farm, the harmful microorganisms represent an increased risk of contracting disease also to neighbouring farms [4].
As the number of fish farms are increasing, the high volume of farmed fish may become disproportional to the corresponding number of natural hosts in a given area. At a certain production level, which may vary from place to place, multiple open net pen farming system run the risk of creating an ecological imbalance in which case a fish farm may become artificial incubators for harmful microorganisms and parasites [5]. Once a fish population is harbouring harmful microorganisms or parasites, it begins shedding to the surrounding environment and neighbour farms. The shedding may expose and affect the net pens adjacent to the diseased fish population, neighbouring sites and potentially also wild salmonids residing in habitats nearby the site. Understanding the exact interaction is challenging and has over many years been subject to substantial scientific research [6].
The Salmon louse is common to farmed salmon. Its reproduction cycle includes both free-living stages and fixed stages in which it resides on the salmon skin. The reproductive capacity increases proportionally to increasing temperature [7] and densities of farms [8]. The Salmon louse is phototactic (migrating towards daylight) and its infective stage behaviour adapts to find a salmonid host predominantly residing in the top layer of the marine environment. It has been suggested that the infective stage of the Salmon louse remains in the first four meters of the surface [9, 10, 11]. Both research and practical farming confirm that infestation levels are significantly less when farmed salmon are sheltered from the top 10 metres exposure of infective salmon louse larvae [9, 10, 11, 12, 13, 14]. However, the use of skirts around salmon cages to reduce infestation of salmon lice, result in reduced oxygen levels and thereby it can stress the fish, impair welfare and feed utilisation [15].
When the salmon louse larvae infects a new host, it lives out of eating mucus, skin tissue and blood off the salmon. The salmon louse may pick up microorganisms and carry for a period of time [16, 18]. Wildlife has many examples of parasites that serve as biological vectors. It is shown that the salmon louse can be a biological vector for microsporidium [17]. Hence, the salmon louse may serve as a mechanical and biological vector that can carry harmful micro-organisms from fish to fish, from one cage to another as well as from fish farm to fish farm.
Salmon louse from salmon farms may affect and harm wild salmonids once shed in high numbers from salmon farms. Especially when the young salmonids are migrating from rivers to the ocean and pass nearby dense farming areas, the risk of negative impact is increasing. Likewise, sea trout populations do have their summer habitat in fjord and coastal areas where they may be exposed to Salmon lice during spring, summer and fall [19, 20, 21]. The spread of sealice, both magnitude, dynamics and pattern is crucial to understand how the challenge can be mitigated, and it is subject to intense research [22].
To protect the welfare of the farmed salmon and the wild salmonids, Government has enacted legislation to keep the level of sealice low in salmon farms, especially during the spring migration period. Since 1988 salmon lice has been treated by use of chemical drugs like organophosphates, pyretroids, emamectin, teflu, -diflubenzuron, hydrogenperoksid as well as combination of these. Since the very start of combating the Salmon louse with chemicals, it has shown a remarkable ability to develop resistance against any drug available. Since 2007, the salmon farmers along the Norwegian Coast have experienced that treatments against the salmon louse have become less effective. Over the last 7 years situation has impaired are currently seeing multi-resistance i.e. no drugs are effective any more. In parallel, use of non-medical tools against the Salmon louse has accelerated. For instance use of cleaner fish have increased substantially [23]. Cleaner fish is fish that eats the Salmon louse off the skin of farmed salmon. This habit is observed also in nature and an elegant way of delousing farmed salmon in a pen. Wrasse was introduced as cleaner fish the nineties. The fish were caught by locally and delivered to fish farms. Industry started to research farming of wrasse in 2009. In 2011, use of lumpfish was introduced as cleaner fish and has become popular due to its higher activity at lower temperatures. Lastly, a wide range of physical and mechanical methods have been tested to alleviate the dependency of drug use. Some of these demonstrate promising results. The advantage of using non-medical tools against the Salmon louse is that these do not generate resistance.
Still, in 2015, the salmon lice represent the biggest fish welfare and environmental challenge for the industry and has far reaching economical consequences [19, 24]. The combating of the salmon lice continue to be predominantly handled by chemical methods and the use of drugs are increasing. Supplemental to this, one is aiming to scale up use of cleaner fish as well as other non-pharma tools.
Since 2009 the cost of combating sealice has risen from NOK 0.50/kg to NOK 5.00/kg and above. The problem of salmon lice is now so serious that the Norwegian Government has decided to restrict industry growth in areas where the salmon lice problem remains unresolved. Future growth will be based on strict performance regarding sealice levels [25, 26].
In traditional net-pen farming of Atlantic salmon, after all fish are harvested, the site has to be fallowed for 2 months before new fish are allowed to be put in. The fallowing period occurs every second year corresponding to the production period of 14-22 months in the sea. The fallowing regime is a sound practice adopted from agriculture and enables the site to cleanse and the seabed surrounding the farm to restore its original state after the farming production period with high organic load due to feed spill and faeces from the fish [27].
In some areas subject to severe sealice burden, a mandatory zone fallowing of 1 month for all sites applies every second year as part of the two months fallowing of individual sites [28]. In fact, due to under-performing sealice management, some sites have been enforced by regulation to reduce the production [29].
While having fixed assets like for instance a barge and numerous large cages sitting empty in a non-productive site, the fallowing periods truly represent an extra cost.
Open net pen farming has during the last decades relocated to more exposed sites with better water current conditions, which allows oxygen rich water to pass through. Consequently, the farmer can hold more fish per site. A well-located site can offer higher volumes of water passing through per unit of time compared to previous sites. But the increased total flux of water may also cause problems. Assuming a random distribution of potential harmful microorganisms in the sea, the total exposure will correspond to the volume of water flowing through a fish site population. So does also the shedding [8]. Even in sites with improved natural conditions, one may suffer disease and parasitic infections. Although natural farming conditions have been much improved by the relocation of sites, the mortality during one production cycle has not improved correspondingly, and is averaging between 10-20% per cycle across the Norwegian salmon farming industry. A recent study carried out by Norwegian Food Safety Authorities following 307 million fish from entry to harvest, concluded average mortality was 16.3% for Atlantic salmon and 18.3% for Rainbow trout [30]. Mortality in fish farms may have numerous causes, for instance infectious diseases, production diseases, loss when handling and fish stress. The study above concluded that issues related to osmoregulation at transfer and infectious diseases constituted the major causes of mortality.
The open net pen systems show rapid variations in temperature, salinity, current, presence of algae, and occurrence of predators (wildlife that see farmed fish as prey). As many fish are unable to adjust to the various stress factors, welfare of the farmed fish is under pressure and elevated mortality is the result. Fish subject to stress, become more susceptible to infectious diseases.
Farmed fish are fed extruded and pelleted feed. These are condensed and high-energy particles ranging from 3-12 mm in diameter. The feed is offered to the fish in the cage largely by automatic feeders and minor volumes by hand feeding. Cameras are located in many of the pens to monitor and prevent over-feeding.
Adequate feeding in various weather conditions is challenging. It is recognised that between 5-10% of the feed is never eaten by the fish and is discharged into the seabed surrounding the site [31]. The economic feed conversion rate in salmon farms ranges from 1.0-1.4 with an average of 1.15 in statistical review. The undigested part of the feed represent 25% of the weight. Assuming one could capture both feed spill and faeces, this would account for at least 30% of the nutrients of the feed [32]. Cost of feed is the single highest cost and represents between 50-60% of the cost per kilo of farmed salmon. In other farmed species it is similar. There is a significant potential for cost saving and for saving of resources and environment by eliminating the waste.
Fish also produce faeces that is discharged in the environment. It currently represents organic waste. The faeces is rich in phosphorus, which is a scarce resource and in global demand. The fish waste can also be utilized for biogas production and blended with other types of organic offal to become valuable fertilizer. The amount of dry matter from faeces in hatcheries varies a lot depending on the physical quality of the feed, raw materials and size of fish [33]. While discharge is subject to filtering in land-based farming like hatcheries, all of the faeces in sea farms are currently discharged into the water and carried away by the current. Depending on the tide and/or the current, there is little or much spread of the faeces. Scientific studies suggest that discharging of faeces is presently not a limiting factor for the industry as long as it sub-cedes the carrying capacity of the recipient. However, it is a waste of resources which could be better utilized.
Fish escape in the salmon farming industry is recognised as a significant problem. Much resources are spent to prevent escape from the farms and yet the endeavours are only rewarded with partial success. Due to the significant number of farms in operation along the Norwegian Coast (˜600) representing maybe as much as 600 million fish, one should expect more fish to escape. The fish farms are vulnerable to the elements. Escape prevention is high on the legislative and industry agenda. It has led to new technical regulations (NYTEK), and it is subject to close monitoring and investigation of incidents by Directorate of Fisheries [34]. Semi-contained (open in the top) farming units have also suffered structural damage during storms.
Escaped fish may enter the rivers and interbreed with wild salmon stocks, destroy egg nests in the riverbed or potentially transfer disease. The magnitude of the damage to the wild stocks of Atlantic salmon caused by escaped farmed salmon and rainbow trout, is still debated. However, it is undesirable to lose fish from a farm. Equally, it is undesirable that escapees end up in the vulnerable ecosystems in salmon rivers [21, 35, 36]. The unresolved escape issue represents a restriction on the Norwegian industry for further growth. Large sea-areas in the fjords that are ideal for farming, are closed due to the risk of escape.
In conclusion, we can say that current net pen fish farming has a significant and untapped potential for increased feed utilization, reduced environmental impact as well as improved fish welfare and waste recycling management.